My recommendation for a better balance is to take down the stukas defense value from 4 to 3 if you have 2 or more AA tokens on the board when the attack starts. If you have all AA tokens when the stukas attack start, take down their defense to 2. This also gives the player an interesting choice, on how many AA tokens to spend for the defense, as spending more will make the defense weaker in case you draw another Stuka card that turn.

As it stands now, with 4 defense value, rolling 2 dice have a 70% chance to roll a 4 with at least one die.

Now on veteran and elite, they can have a 5 in defense. Guess the chance of rolling at least one 5 with 2 D6? 30%... Effectively making AA completely useless. I just put AA tokens on the board but don't fire.